The present invention relates to vent assemblies and particularly to vent assemblies for venting exhaust gases from fossil fuel fired appliances, such as gas hot water heaters.
A conventional gas hot water heater consists of a hot water tank above a gas burner and disposes of the products of burner combustion through a central flue or through multiple flues that extend vertically through the tank. The water therefore is heated both by the heat of the flame at the bottom of the tank and by the hot exhaust gases passing through the flue or flues. These hot gases exit each such flue at a flue exhaust port and enter, through a draft hood thereabove, a duct leading to the chimney, thus disposing of the products of combustion into the atmosphere above the building containing the water heater.
Where a heater flue and draft hood come together, one or more relief apertures are provided so that the flue is open to the ambient atmosphere within the building. This is necessary for a number of reasons. In the event of a blockage along the conduit to the chimney, caused, for example, by a bird or raccoon nest, the products of combustion that back up in the chimney would otherwise collect within the flue to the location of the burner, displacing the requisite ambient atmosphere necessary for complete combustion, and in extreme cases collect in such concentrations as to create the possibility of explosion. Instead, the relief apertures allow these gases to spill into the building where they are dispersed. A similar risk avoided by the relief apertures is that risk created during the unusual, but certainly not unknown, occurrence of wind blowing down the chimney.
Unfortunately, the apertures cause a measurable increase in fossil fuel consumption due to an increased loss of heat from the water in the tank and due to a movement of room air up the chimney, room air that has been heated at some expense by the heating plant of the building. Furthermore, the loss of air up the chimney tends to create low pressure within the house, increasing the amount of cold outside air that is drawn into the house through air leaks such as those prevalent around windows and doors. This disadvantage is remedied partly by the installation of a vent damper within the venting system. Such dampers are currently available and may be electromechanically or thermally controlled.
Even with the use of a vent damper, there is still, however, a continuing exchange of heat between the water in the tank and the air within the flue. As the air within the flue becomes heated by this exchange, it naturally forms a convection current and exits from the venting system, either through the chimney or thorugh the draft hood apertures if such a vent damper is closed. This allows a free exchange of air between the flue and the surrounding ambient atmosphere until the heated air is replaced by ambient, cooler air. The cooler room air is drawn into the flue, continuing to cool the water. This cooling by the convection current requires an increased frequency of burner operation to maintain a "hot water ready" condition.
A second damper, located at the exhaust outlet of the flue may be provided to alleviate heat loss due to convection currents through the flue passages of the tank, but the controls for a two damper system are, of necessity, complex. The second damper could not be thermally controlled but must, instead, be electrically or mechanically controlled. This is true because the exhaust outlet of the flue must be opened before ignition of the burner to prevent possible explosion or incomplete combustion. What is required, then, is a system with a plurality of moving parts such as gears, motors, wires or linkages.
It thus is desirable and the object of this invention to provide a vent assembly that accomplishes the same reductions in fuel consumption as provided by a two damper system but retains the economy and safety provided by a single damper system and inherent in using a reduced number of moving parts.